Prevention of interference in telephone circuits



May 19, 1942.

PREVENTION OF INTERFERENCE IN TELEPHONE CIRCUITS Filed July 7, 1938 5 Sheets-Sheet l I INVENTOR.

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PREVENTION OF INTERFERENCE IN TELEPHONE CIRCUITS Filed July 7, 1958 I s Sheets-Sheet 2 IN VEN TOR.

ATTORNEY.

I May 19, 1942. F. D. LAYNE 2,283,891

PREVENTION OF INTERFERENCE IN TELEPHONE CIRCUITS Filed July 7, 1938 3 Sheets-Sheet 3 Elm INVENTOR: jZrr-esl J @7712,

' ATTORNEYS.

Patented May 19, 1942 PREVENTION OF INTERFERENCE IN TELEPHONE CIRCUITS Forrest D. Layne, Oklahoma City, Okla., assignor of one-half to J. E. Piersol and J. Enoch Piersol, Jr., Oklahoma City, Okla.

Application July 7, 1938, Serial No. 217,983

8 Claims.

The invention relates to telephone circuits, and particularly to automatic means for preventing interference with such circuits by electromagnetic disturbances induced variously. While the invention is particularly aimed at preventing impositions on telephone receivers of impulses generated by power or other electricity conducting lines adjacent or parallel to the circuit with which the receiver is coupled, it will also serve to a large extent in eliminating static disturbance signals'in the receiver.

It is also another object of the invention to enable the production of a receiver system embodying the invention with a minimum of difficulty and expense for materials and labor in manufacture.

Another important object is to present a device operative for the ends in view with satisfactory efliciency which will be liable in a minimum degree to derangement and which will not involve special expense for maintenance.

A further object is to present such a device the construction and theory of which will be readily understood by workers in the art and which may be readily repaired by them in case of damage, or need for replacement.

It is also an important object to present such a device which may be adjusted to various situations where interference occurs, without requiring special manufacture of different forms of the article, but permitting adaptation by adjustment of elements of a standard unit adapted to be included in various service installations.

Another aim is the provision of a novel transformer for such uses and a hook-up relation thereof to a telephone circuit that is novel in structure and function.

An important aim of the invention is to present a construction which may be embodied in an accessory unit attachable to, or adapted to be incorporated in, conventional telephone sets in use or manufactured heretofore.

Additional objects, advantages, and features of invention reside in the construction, arrangement, and combination of parts involved in the embodiment of the invention as well as in the system, form, and relation of the elements, as will variously appear in the following description, and accompanying drawings, wherein Figure 1 represents a common local battery telephone, customarily equipped with a magneto.

Figure 2 illustrates an auto transformer coil of novel construction.

Figure 3 is an alternative construction of the coil.

Figure 4 shows the coil applied to the telephone circuit to improve reception.

Figure 5 is an improved noise suppression circuit, wherein a condenser is inserted between coil and receiver.

Figure 6 is a further improvement wherein series capacitance is inserted between the line and the coil.

Figure 7 shows a different mode of connection of the elements of the circuit.

Figure 8 is a further modification wherein capacitance is inserted within the secondary of the auto transformer coil.

Figure 9 presents a form wherein a choke coil is shunted across the receiver to by-pass low frequencies.

Figure 10 is a circuit employing a part of the usual transformer as an impedance in a tuned receiver circuit.

Figure 11 is a diagram of an eliminator applied.

to a line;

Figure 12 shows a modification of the coil device of Fig. 11,

Figures 4 to 8 inclusive, each shows a transmitter-receiver unit adapted to be installed as a 7 single station device.

The customary local telephone circuit is illustrated in Figure 1, as a basic circuit to which my improvements may be applied. It will be apparent, however, that the invention is equally applicable to various other instruments and circuits, including the latest modern equipment, and the invention may also be incorporated as part of the original manufacture in new apparatus conforming to present day practices in telephonic instruments and circuits, with or without the local battery and/or magneto. The showing mentioned includes a local battery connected in series with a transmitter 2| and the primary 22 of a transformer whose secondary 23 is connected in series to a line 25 -26. The line wire 26 is interrupted at binding posts 29 across which the conventionalreceiver 28 is ordinarily connected, but which are differently utilized in my invention. Suitable switches and a magneto 21 are provided to actuate call bells and control the circuits but are not essential to the function of my improvements.

A simple embodiment of my invention is shown in Figure 4, in which a coil 30 of special characteristics and construction to be described, is shunted across the receiver, being directly connected at 3| to the line 26, and a lead from the outer line 25 being tapped in at a medial point 32 of the coil. The coil 30 is shown in detail in Figures 2 and 3, and it will be noted that an iron core 33 extends entirely through a portion 34 of the coil of Fig. 3 which may be termed a secondary, but extends only part way into a primary portion 35 of Figs. 2 and 3. Taps 43 are provided upon coil 30 by which the proportioning of the number of turns in the primary and secondary may be varied to suit the requirements of respective installations. Or the volume of iron Within the primary turns of the coil may be diminished by tapering its cross section. The dimensions of the coil, the amount and distribution of iron in its core and the relative placement of the tap 32 must be governed by the characteristics of the associated circuit. However, I have used a coil of 1200 turns, the primary portions 35 embracing 300 turns and in which the core extended half-way into the primary, i. e., a

distance of about one and one-half inches.

This is the simplest embodiment of my invention, and is adapted to use where mild low frequency interferences are to be remedied, without material high frequency interference. cial design of coil produces a difference in the characteristics of its primary and secondary portions which results in reducing the low frequency interfering noises. It seems that reducing the amount of iron in the primary makes its resistance to low frequencies less, relative to its resistance to high frequencies; thus, the low frequencies tend to be shunted therethrough without transformation, while the higher frequencies are effective upon the secondary and are stepped up by induction. At the same time the secondary is of high impedance, due to its iron core, and tends to choke out those low frequencies which would otherwise be conducted through it, although not transformed.

Where more positive means for stopping low frequency interference is required, I may interpose a condenser 36 between the receiver and the secondary 34 of the coil 30, as in Figure 5. The selective reactance of the condenser relative to high and low frequencies blocks the low noise frequencies and permits only the higher voice frequencies to affect the receiver. The circuit may preferably be adjusted to have a minimal impedance in frequency range of 1000 to 1100 cycles.

As shown in Figure 6, the efiiciency of capacitative action may be increased by incorporating in the plan of Fig. 5, a condenser 31 between the tap 32 of the coil 30 and the line 26'. this permits adjustment of the circuit characteristics relative to the coil 30 which attain the optimum conditions of noise suppression.

In the fore described connections, it will be noted that my invention may be applied across the usual binding posts 29 which ordinarily supply the receiver, and that the receiving means are therefore in series with the transformer winding 23. This results in the receiver being not quite so directly responsive to modulations of line voltage, as these must suffer some impedance drop in the winding 23 before reaching the receiving system. Figure 7 illustrates a connection of my modified receiving means across the line, paralleling the winding 23 and therefore having impressed upon it the full line voltage modulations. The binding posts 29 are in this case shorted out by the conductor 38, or a condenser as at 39, Figure 8, may be used to further impede the low The spe- Again 5 frequency disturbances while readily passing the voice modulations.

A circuit embodying an effective modification of the coil 30 is shown in Figure 8, wherein the secondary winding 34 is split and a capacitance 49 is inserted. Use of this arrangement permits impedance to vbe matched and increases efficiency. This cooperates in the effect of the modified characteristics of the secondary relative to the primary 35 upon the high voice currents and the low noise disturbances, tending still further to confine the noise impulses to the primary while permitting the higher frequencies to be more readily carried and transformed in the secondary. Harmonics of 60 cycles are eliminated up to the third harmonic.

A further refinement is shown in Figure 9 comprising a choke coil 42 connected across the receiver which tends to shunt the low frequencies without letting them affect the receiver. This effect is aided by the condenser 36 which is in such proportions that the circuit is most compliant to frequencies of about 1000-1100 cycles.

A connection is illustrated in Figure 10 which utilizes a portion of the usual transformer as a corrective shunt in conjunction With a condenser 41 which tunes the circuit to the desired response. The receiver 28 and condenser 41 are connected in series between the line 25 and a medial tap of the winding 23 of the transformer.

The eficacy of the circuits of Figs. 5 to 10 seems dependent upon the peculiar characteristics attained in the auto transformer coil 30 due to construction with a designedly lesser volume of iron linking the primary to the secondary than the volume of iron enclosed by the secondary, and to the insertion of capacitance in the circuit within the secondary winding. The result of this distribution of iron in the core is that the primary portion, containing least iron, has little impedance to low frequency noise currents, and is only loose coupled to the secondary. The higher frequencies in the range of voice currents are passed by transformer action, aided by the interposition of the condenser in the secondary winding, while the low frequency current disturbances are impeded from conduction through the secondary by its high impedance at their frequencies, and are not materially transformed due to the loose coupling of the coils. Transmission losses are minimized by use of capacitance to permit matching of impedance, which may be readily done through the adaptability of the elements of the circuit.

Figure 11 illustrates a device which is adapted to the control of definite local fields of interference such as now arise from an adjacent power line or other causes. The live side of the power transmission line induces a widespread magnetic field, which may link the circuit of the telephone transmission line and induce therein very objectionable electrical current impulses.

It is apparent that a point may be selected in the telephone line where the interference field linking one portion of the telephone line from that point is equal to that linking the portion of the line in the other direction. The interferencefield-induced voltage in each part of the telephone line will here be equal. If the line be cut at this point and the induced voltage to ground be measured for each cut end, these voltages will be found equal, but oppositely directed.

Having discovered this point, symmetrically central to the disturbing field, I insert a transformer 5B in the line, the primary 52 and secondary 53 being connected in series and being grounded at their point of connection. An alternative structural form of this transformer is shown in Figure 12 wherein the mid point of a coil on a cylindrical core is grounded. The coils of the transformer must be connected in such sense that a current in one coil, flowing to the ground will induce a similar current in the other coil also flowing toward the ground.

Referring to Figure 11, the telephone station 65 is the source of a telephonic voice current Iv carried by the line 58 which passes through the coil 52 of the transformer 50, into the ground 55 and completes its circuit to the station 54. An induced voice current Iv is produced in the coil 53 of the transformer and makes its circuit to station 56 by way of the ground and the transformer line 50. Thus speech impulses are conveyed between the stations 54 56. The loss in speech transmission is about 0.75 decibel, andv cannot be detected by the ear. The coil used may have an ohmic resistance of 24 ohms and an impedance at 1000 cycles of 1000 ohms.

The action of the device in preventing interference from the power line 60 may be understood by considering the currents In and InZ to be the noise currents which tend to be induced in lines 58 and 59 respectively. An induced current I'n, would appear in the coil 53 and an induced current I'nz would appear in the coil 52. However, by the connection of the coils, these transformer-induced currents are equal and in opposition to the currents induced from the transmission line. Hence, no noise currents flow.

This may also be understood by considering the voltages to ground induced by the changing flux linkage between the transmission line 60 and the respective telephone lines 58 and 59. These voltages En, and E112 respectively, are equal and are oppositely directed with respect to ground. However, the transformer induced Voltages En and En'z are also equal and oppositely directed in such sense as to oppose the voltages En and E112. There being no resultant noise electromotive force there is no noise current, nor interference.

Since the essential feature is the balancing out of a transmission-line-induced voltage by a transformer-induced voltage, it is possible, by varying the ratio of transformation, to balance out unequal induced voltages to ground. However, this would not be so efficient in the proper transmission of voice currents, and I prefer to use a 1:1 ratio of transformation, inserting the device symmetrically with respect to the induced disturbance.

I am aware of the patent to Condon and Barrett, No. 910,176; the practice of using condensers in telephone circuits because of their characteristic of passing high frequency currents better than low frequency currents; and also that cored coils have been used in such circuits because of their property of retarding sharp current fluctuations, high potential surges, and extremely high frequency currents, and do not seek to cover such elements otherwise than as claimed herein.

I claim:

1. In a telephonic system, a transmission circuit, an iron cored coil, a portion of the winding constituting the primary thereof connected in series to the transmission line, a receiver connected in parallel with the entire coil whereby this constitutes a secondary in series with the receiver, the iron core of said coil being of reduced inductive effect in the primary portion of the coil, and extending throughout the remaining portion of the coil with fully effective structure.

2. The structure of claim 1 wherein one coil of the usual transmitter transformer is in series with the line and primary portion only of the first named coil.

3. The structure of claim 1 wherein capacitance is placed in series with the receiver and capacitance is placed in series between the common terminal of primary and secondary of the coil and the transmission line.

4. The structure of claim 1 in a transmitterreceiver wherein capacitance is placed in series with said receiver, a capacitance is inserted between the transmission line and the common junction of primary and secondary of the'coil, and a coil of the usual transmitter-transformer is in parallel with the condenser-and-primary portion of said coil.

5. The structure of claim 1 wherein a coil of the usual transmitter transformer is in parallel with the primary portion of said coil, capacitance is inserted between transmission line and the receiver terminal connected with terminal of the primary portion of said first named coil, and capacitance is inserted intermediately of the secondary Winding of said coil.

6. The structure of claim 1 wherein a coil of the usual transmitter transformer is in parallel with the primary portion of said coil, capacitance is inserted intermediately of the secondary portion of said coil, a choke coil is placed in parallel with said entire first named coil, and capacitance is placed in series with the receiver.

'7. An interference eliminator for telephone lines and the like consisting of a voice current transmission line, a known source of interference-producing magnetic radiations, a transformer having coils mutually connected at one end, said transmission line being broken and its end portions at the break connected respectively to the remaining opposite ends of the coils, the mutually connected ends of the coils being connected to the opposite side of the said transmission line, the ratio of transformation of the transformer being proportionalto the intensity of the voltages induced by the said radiations in respective portions of the transmission line on each side of the inserted transformer, whereby impulses generated by said radiations in each of said portions of the said transmission line arecounteracted by voltages induced in the coil connected to the opposite of said portions of the transmission line by the same radiations.

8. The method of correcting undesired interference signals in telephone circuits consisting in determining a point in the field set up across the telephonic transmission line by the interference producing radiations such that the interference voltage in the two adjacent portions of the line are of equal magnitude, severing the telephonic transmission line symmetrically with respect to said field, connecting the respective portions of the severed line to the ends of transformer coils and mutually joining and connecting the opposite ends of the coils to the opposite side of the circuit, said coils being electromagnetically coupled in such manner that the magnetomotive forces due to a series current therethrough would be in the same sense.

FORREST D. LAYNE. 

